The use of polychlorinated biphenyls as dielectric fluids, even in sealed electrical equipment, may become very restricted because they are alleged environmental pollutors, which is aggravated by their low biodegradability. Efforts during the past few years to develop dielectric fluids that could replace trichlorobiphenyl as the impregnant of polypropylene-film-paper and all paper capacitors and to be used with 100% film have been directed mostly at materials with aromatic groups. Highly aromatic fluids have been considered as alternates to permit continued operation of the capacitors at high voltages because they have good corona properties, and the operational voltages of a power capacitor depends on its resistance to corona generating overvoltages. Examples of potentially good power capacitor fluids are solutions of a phthalate ester, diisononylphthalate, and an aromatic, solutions of an aromatic hydrocarbon and an aromatic sulfone, and isopropylnaphthalene, which is used in Japan.
These fluids are biodegradable, but do not have the excellent resistance to combustion, the "non-flammability," that polychlorinated biphenyls have. But their flash and fire points are as high as that of mineral oil which is widely used as an electrical insulating fluid. They would not be considered to be serious fire hazards in most power capacitors, which are usually mounted outdoors, because of the small volume of fluid per unit, less than 3 gallons, the fire safety limit for such fluids.
The emphasis on high operating voltage stresses is particularly significant for the KVAR rating of film-paper capacitors, where the magnitude of the dielectric constant of the impregnant is not too important. The KVAR rating is proportional to the product of the square of the operating voltage and the first power of the capacitance. In the film-paper dielectric the capacitance is only slightly affected by the dielectric constant of the impregnant, as it is dominated by the film, whose dielectric constant is not changed much by the impregnant because only a small amount of fluid is absorbed by it. The average dielectric constant of a 75% film and 25% paper dielectric is decreased by only 105 by changing the impregnant from one with a relative dielectric constant of 4.9, trichlorobiphenyl, to one with 2.2 to 2.6, a hydrocarbon. On the other hand, for an impregnant that permits a modest increase in operating voltage stress, the KVAR rating is increased by about twice that fractional voltage stress increase.
The level of the rated stress is based on the expectation that a power capacitor will be subjected periodically to high overvoltages, due to switching and certain transients in the power lines, which are of the order of up to three times the rated voltage. It must resist the effects of such overvoltages, which generate corona discharges in the fluid, in two ways. One is that corona cannot persist, that it extinguishes, after the rated voltage is restored. Secondly, the corona at the overvoltage should not damage the dielectric and lead to early failure, before the 20 to 30 years of required operating life of the capacitor. Such effects in a capacitor may be gauged by its corona discharge inception and extinction voltages, which are determined by the nature of the impregnant, where the corona occurs, and a proper selection of capacitor dielectric spacer arrangements and foil electrode geometry.
It is not very clear why aromatic fluids have relatively good corona properties, especially compared to aliphatics. Their good corona properties are evidenced by high capacitor corona discharge inception, and extinction voltages, and relatively low gassing tendencies of the liquid under high voltage, in tests such as the modified Pirelli Gassing Test (ASTM D2300). A similar situation exists regarding additives, such as anthraquinones and epoxies, that improve the corona properties of fluids. Regarding resistance to the effects of corona, high extinction voltage and low gassing, it is suggested that the aromatic molecules or constituents, or additives, react with the products of corona discharge, preventing build-up of gas bubbles of hydrogen and hydrocarbons at the original site of the corona, so that corona may not persist there. (A similar suggestion has been made about the high corona discharge extinction voltages with polychlorinated biphenyls, that their corona products, such as hydrogen chloride, are soluble or reactive.) This general explanation is not sufficient to present a systematic order of resistance to corona, and to use as a basis of selection of corona resistant fluids. As for the magnitude of the corona inception voltage, the molecular factors that affect it are also quite unclear.